1. Field of the Invention
Implementations of various technologies described herein generally relate to methods and systems for water table depth mapping.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
The depth of the water table is useful for seismic exploration because the water table generally represents the first seismic event. The air in the pore space of the rock near the surface above the water table is replaced with water, which usually results in a significant increase of the acoustic impedance and hence a reflection. The water table may also represent the first refractor. For seismic imaging, information about the depth of the water table below the surface may be of great importance. The information about the thickness of the weathering layer, i.e., the rock or soil layer between the surface and the water table, may contribute to the statics corrections.
State-of-the-art methods for measuring water tables depend on the availability of drilling or geophysical equipment on the ground to obtain information about the water table depth. This approach is often not possible in remote swampy terrain, such as the Arctic. Furthermore, the acquisition process may be limited to the acquisition of 2D line profiles, which results in considerable time and effort for mapping the water tables in large areas.
Presently, water table measurements are carried out for ground water management and geotechnical projects. The water table may be either directly measured from invasive methods, such as, water well drilling, or inferred using geophysical methods. Geophysical methods are generally non-invasive and hence, more environmentally sustainable than drilling. Table 1 below provides an overview of the most common methods for water table mapping.
TABLE 1MethodTechnologyDeliverableInvasive methodsWater WellMechanical1D point profilesDrillingprobingGeophysicalGPRMicrowaves2D line profilesmethodsGeoelectricEM conductivity2D line profilesSeismo-electricElastic-EM2D line profilesconversionSeismicElastic waves1D, 2D, 3D profiles
Geophysical methods include ground penetrating radar (GPR), geoelectric methods and seismic methods. Ground penetrating radar emits microwave radar pulses, the scattering of which at the water table is interpreted to map its depth. Geoelectric methods measure the electric or electro-magnetic properties of the subsurface and interpret the response of the change of these properties at the water table for its depth. Geoelectric methods use either the self-potential (passive method) or the earth response upon an induced electro-magnetic signal (active). Seismic methods employ elastic waves and map the arrival of their reflection or refraction at the water table to infer its depth.